This invention relates to a process for making clad articles of densified metal powder, and in particular to such a process for making a clad article having improved workability and formability as a result of the improved preparation technique.
U.S. Pat. No. 4,259,413 ('413 patent), R. J. Taglang and W. C. Ziolkowski, issued Mar. 31, 1981 and assigned to the assignee of the present application, discloses a clad article that includes a core of densified metal powder and a metal cladding that is compatible with the metal powder. The process disclosed for making the article includes compacting a metal container filled with prealloyed metal powder. The compacted, powder-filled container is then hot and/or cold worked to form a shaped, clad article.
The '413 patent stresses the need to prepare the interior of the container properly before the powder is added and points out that cleaning with a solvent to remove foreign matters, though desirable, is not sufficient to remove adherent material or coatings including oxides. To remove such materials, particularly oxides, the '413 patent teaches the use of an organic solvent, followed by chemical, e.g., acid cleaning, or by mechanical cleaning, as by sanding or sand blasting. Any suitable technique for filling the containers with the metal powder can be used as long as the powder entering the container is free of adsorbed water. Vacuum filling in which the metal and the container interior are maintained at about 10 microns Hg is specified. Alternatively, metal powder that has been thoroughly dried, as by heating in a fluidized bed, may be filled in dry air or in a dry inert gas at atmospheric pressure. After air and water vapor have been eliminated, the container is sealed and then compacted.
U.S. Pat. No. 4,891,080 ('080 patent), G. J. Del Corso, J. W. Martin and D. L. Strobel, issued Jan. 2, 1990 and assigned to the assignee of the present application, relates to a workable, boron-containing, stainless steel article and the process for making such an article. The '080 patent discloses a powder metallurgy technique in which the metal powder is baked to remove moisture prior to being loaded into a similarly baked canister for compaction. The metal powder and the canister are baked at less than 400 F. to avoid oxidation. The '080 patent points out at column 5, lines 1-2 that the canister "must be clean and essentially free of oxides."
The teachings of the referenced patents have been used successfully to produce relatively small, clad articles containing less than about 400 pounds of metal powder in which the metal cladding is bonded to the densified metal powder core. The present invention stems from the discovery that, in such articles, metal oxides are inevitably present in a zone of the core adjacent the core/cladding interface. Here and throughout this disclosure, the terms "oxide" or "metal oxide" refer to any oxide of metals such as Mn, Cr, Ni, Fe, etc. When significantly larger intermediate articles such as billets or slabs, containing about 400 pounds or more of metal powder are made by such processes, the presence of a significantly larger concentration of such metal oxides in a zone of the core extending a limited distance from the interface toward the center of the core as compared to the remainder of the core material, results in significantly reduced local ductility compared to the remainder of the core material. Such reduced local ductility has adversely affected the workability of the clad article in such operations as forging or rolling, and would adversely affect its formability in such operations as drawing or bending.